It is known that treatment of chrysotile asbestos with phosphorus oxychloride leads to the reduction of physiological activity of the treated fibers (U.S. Pat. No. 4,356,057 Lalancette et al.). In this patent little consideration is given to the recovery of the gaseous phosphorus compounds nor to the obtaining of maximum yield. It is an object of the present invention to provide an improved phosphating process whereby the reaction of the phosphating gas is substantially complete and whereby any unreacted phosphating gas is readily recovered.